Prediction of response to chemotherapy before treatment by study of a patient's tumor in vitro is a major unrealized goal of cancer research. The objectives of this study are to develop an assay using cultured tumor tissue that can reliably predict drug resistance, and assess its clinical applicability. Human tumors with known drug response pattens will be maintained as xenografts in athymic mice and used as sources of tumor tissue for validation of the assay. To estimate the range of responses in vitro to several widely used anticancer drugs, the assay will be performed on a series of human tumors obtained at the time of surgery. Intact cultured tumor tissue has practical advantages as a test material over the more commonly used tumor cell suspension. However, analysis of drug response in cultured tissue has been difficult. This study will develop a new method to make analysis easier and more exact. Tumor explants will be doubly radiolabeled in vitro, by incubation with 14C-thymidine before, and with 3H-thymidine after, drug exposure. 3H/14C incorporation ratios from parallel untreated control and treated tumor explants will be plotted as a function of drug concentration. In preliminary studies, smooth drug response curves can be obtained with small tumor samples. Including high non-pharmacologic drug concentrations in the assay should allow definite identification of many drug resistant tumors. Tumors that maintain DNA synthesis after exposure to a drug at many times the peak plasma level will probably not respond to that drug in vivo. Use of ineffective anticancer drugs is very costly and produces toxicity without benefit. Identification of these drugs before treatment should reduce health care costs for cancer patients.